SHOWN HERE is a grapefruit rootstock trial with trees infected with citrus greening that is being studied by University of Florida Institute of Food and Agricultural Sciences researchers with the UF/IFAS Citrus Research and Education Center in Lake Alfred. The tree in the foreground, although infected with citrus greening, is healthy in appearance, while the trees next to it are also infected but are in poor shape.

• The UF researchers have identified citrus cultivars, in this case 16 citrus rootstocks, most of which show a lower rate of infection and more tolerance to citrus greening.

By Robert H. Wells, University of Florida | Sep 27, 2013

University of Florida researchers have found encouraging results in the ongoing battle against citrus greening disease.

The UF researchers have identified citrus cultivars, in this case 16 citrus rootstocks, most of which show a lower rate of infection and more tolerance to citrus greening — the dreaded disease that has devastated Florida’s citrus industry since its arrival in 2005.

Growers use citrus rootstocks, part of a plant that includes a portion of root, as a foundation to graft new trees, as opposed to growing them from seed.

For now, data on the rootstocks are limited, but the scientists will gauge their potential in test plots, and UF fast-tracked their release this summer for large-scale testing by an industry in dire need of solutions.

Greening has cost Florida’s economy an estimated $4.5 billion in lost revenues since 2006 and poses a huge threat to the state’s $9 billion citrus industry, the nation’s largest. It weakens and eventually kills infected trees.

Greening has also spread to the country’s other big citrus-producing state, California, where it was detected in 2012.

Large-scale trials of the promising rootstocks could begin in March 2014, and it could be another three to five years before they are available to growers, said Jude Grosser, a horticulture professor with UF’s Citrus Research and Education Center in Lake Alfred.

Grosser and Lake Alfred colleagues Fred Gmitter, a horticulture professor, and Bill Castle, a horticulture professor emeritus, have led the effort to develop the rootstocks. They are all faculty members in UF’s Institute of Food and Agricultural Sciences.

“What’s happening is fields are becoming living laboratories now because the greening disease is spreading so quickly,” Grosser said. “Some people have estimated that 70 percent of the trees in the entire state are infected now, and it’s predicted to go up in the 90s in a very short time.”

Disease is challenging everything

“Everything’s being challenged by the disease, and we’re seeing differences in the rootstock material in the field,” he said. “We’re seeing very genetically diverse material interact with the diseases, and some things are reacting better than others.”

In some cases, the researchers have seen commercial rootstocks with 70 percent infection rates next to experimental varieties that are only 10 to 20 percent infected, but still producing fruit, after four years of exposure to the disease. Whether the trees grown on the less affected rootstocks will survive and remain productive is something the researchers will continue to monitor.

One of the reasons greening has hit the citrus industry so hard is that growers depend on just a few varieties to produce their crop, because they’re most likely to withstand Florida weather and still make fruit consumers want to buy, Grosser said.

“For the course of the last several 100 years, we’ve been eliminating the genetic diversity in the wild, so citrus has kind of gone to a monoculture where there are just a handful of varieties grown,” Grosser said.

“So you’ve limited your ability to adapt when there is a new pressure that comes along.”

Gmitter said it’s fortunate the researchers have maintained a diverse selection of citrus varieties for just such an occasion.

“We took that approach from the beginning to try and get as much genetic diversity into our breeding program as possible,” Gmitter said. “You don’t know what the future is going to bring or what the future problems are going to be.”

Gmitter said he is cautiously optimistic about the rootstocks and that they need to be validated on a large scale.

“They may be part of an integrated solution to citrus greening that includes managing psyllids, the insect that spreads greening, and improved nutritional programs to keep citrus trees healthy,” he said. “All of those things together might contribute to an effective way of managing this disease and remaining profitable and keeping this industry alive.”